The invention lies in the field of leather processing. More specifically, the invention relates to a full grain napa cowhide, one side of which is provided with a finish that partially comprises a foam structure formed from a stabilized polyurethane dispersion containing hollow microspheres. The invention furthermore relates to a method for fabricating such leather.
Such leathers are already known. German patent DE 24 45 605 C3 and the corresponding U.S. Pat. No. 4,016,326 (Schxc3xa4fer), for example, disclose a leather for producing shoes, cushion covers, or the like, which is provided with a porous elastomer coating that contains microspheres whose shell consists of polyvinylidene chloride copolymerisate or polyvinyl chloride copolymerisate and contains an interior gas. The surface of the coating that is averted from the leather can be additionally provided with a finish.
U.S. Pat. No. 4,751,116 (Schxc3xa4fer et al.) discloses the application of a coating consisting of two layers on the fibrous surface of split leather. The layer facing the surface of the split leather is formed by a stabilized polyurethane dispersion containing hollow microspheres. The surface of the coating that is averted from the split leather can be additionally provided with a finish.
U.S. Pat. No. 4,923,732 (Schxc3xa4fer) likewise describes a split leather which is provided with a coating of two layers on its fibrous side. The layer facing the split leather includes hollow microspheres.
In the automotive industry and in the fabrication of high-quality upholstered furniture, as well as for shoe upper material, full grain cowhide is used, which has the necessary characteristics and which can have only a very thin finish in order to be classified as real full grain cowhide. The leather must exhibit the requisite elasticity and have a softness grade of at least 4, preferably better than 4.5, so that on one hand it can be formed into the required shape, and on the other hand, it guarantees a pleasant sitting sensation and a pleasant reaction to touch.
The softness grade is measured with a softness tester BLC ST300 according to IUP36 given a ring diameter of 25 mm. Such softness is achieved by extra long milling (fulling) of the leather skins. But in the process loose grains emerge on large parts of the leather skin equaling between 25% and 40% of the overall surface of the skin, for instance in the stomach region. This area is unsuitable for processing the leather for high-quality auto interiors such as seats or for upholstered furniture or high-quality shoes; therefore, a large amount of expensive leather skin is wasted by extended milling. Another disadvantage of using the known leathers which are provided with a finish is that they are insufficiently breathable, if at all.
It has therefore been proposed that a plastic material that has been foamed by stirring in air be applied on the grain side of the leather and covered after being stabilized by a varnish layer which undergoes embossing in order to form a grainy surface. But the foam material is pressed together in the region of the grain valleys, so that the softness and elasticity which is produced by the foam is lost there. The foam layer thus has different thicknesses; namely, it has a lower thickness on the peaks of the grains, which are more heavily worn with use, which is disadvantageous for later utilization. Besides this, with this known configuration the danger exists that the grain structure may be lost when the leather is stretched over edges or curves or the like owing to the pliability of the foam layer, above all under the influence of heat.
It is accordingly an object of the invention to provide a full grain napa cowhide leather and an associated production method which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which further improve a fully grained napa cowhide such that it has the characteristics required in the automotive and upholstery industries and above all has no notable grain pipeyness after the required milling and does not lose its grain stability even with expansion, and which has the required permeability to water vapor. In particular, the inventive leather should be breathable, wearproof even with respect to its color, lightproof, hydrolysis resistant, and scratchproof. The leather must also be fogless and have low perspiration, so that when it is utilized for auto furnishings the inside of the windshield does not steam up. Also, above all, no toxic gases must be released. The surface of the leather must also be resistant to certain chemicals as well as water, and the leather surface which is provided with the graining should not change disadvantageously when applied over base materials, for instance when the leather is stretched around curves or edges.
With the foregoing and other objects in view there is provided, in accordance with the invention, a dressed leather product, comprising:
full grain cowhide having a grain side;
a finish provided on the grain side, the finish including:
a first layer with a thickness of between 0.015 mm and 0.04 mm applied on the grain side of the cowhide and formed of a stabilized dispersion of at least one material selected from the group consisting of polyurethane and polyacrylate and having a primary proportion of hollow microspheres with a diameter of less than 45 xcexcm forming closed cells and a proportion of at least 10% by volume-of open cells; the first layer having an embossed top surface; and
a second layer disposed on the first layer and formed from a cross-linked polyurethane-based varnish application containing a dulling agent;
and wherein the leather product undergoes a milling treatment.
In accordance with an added feature of the invention, the first layer is a pigmented layer, the microspheres have a diameter of substantially between 15 xcexcm and 35 xcexcm, and the second layer is a multi-coating layer.
With the above and other objects in view there is also provided, in accordance with the invention, a method of producing a full grain napa cowhide. The method comprises:
providing cowhide material having a grain side;
providing a first dispersion based on at least one material selected from the group consisting of polyurethane and polyacrylate, the dispersion containing at least one of hollow microspheres with a diameter between 15 xcexcm and 45 xcexcm and compact particles from which the microspheres are formed in situ by heat addition, the microspheres having a thin shell of polyvinylidene chloride and polyacrylnitrile containing a heavy gas;
applying the first dispersion on the grain side of the cowhide, and thereby selecting a quantity of the dispersion such that, after the dispersion is stabilized by dehydration, a first layer with a thickness between 0.015 mm and 0.04 mm is formed, the first layer comprising primarily closed cells formed by the microspheres and at least 10% open cells by volume;
embossing the stabilized first layer with one of pressure and heat processing;
applying a cross-linkable polyurethane dispersion on the first layer to form a second layer; and
subjecting the leather to a milling treatment.
The term full grain napa cowhide refers to a leather which acquires its softness by extended milling, during which the milled napa surface structure with the characteristic xe2x80x9cmilling grainxe2x80x9d develops. This type of leather can only be produced inadequately by known techniques if at all.
According to the invention, the object is achieved in that the finish consists of a first, preferably pigmented layer with a thickness between 0.015 mm and 0.04 mm on the grain side of the full grain napa cowhidexe2x80x94which consists of a stabilized dispersion containing polyurethane and/or polyacrylate and comprising primarily hollow microspheres with a diameter of less than 45 xcexcm (preferably between 15 xcexcm and 35 xcexcm) forming closed cells but also a portion of at least 10% by volume of open cells, and comprising an embossing on its top surfacexe2x80x94and a second, potentially multi-coating layer on this first layer, which is formed from a cross-linked varnish application containing a dulling agent; and in that the leather undergoes a milling treatment. The second layer consists of a stabilized diluted polyurethane varnish application.
According to a separate feature of the invention, the hollow microspheres have a thin shell which consists of at least 75% polyvinylidene chloride and less than 25% polyacrylnitrile, and they contain a heavy gas, preferably isobutane, in their interior.
The interspaces between the hollow microspheres form the open cells which are needed for the water vapor permeability and breathability. These open cells can be further multiplied by inventively opening the thin shells of individual hollow microspheres by mechanical and/or chemical means, so that individual hollow microspheres thereby also form open cells. To accomplish this, the second layer can inventively contain a solvent such as ethylacetate or methylethylketone which partly dissolves the thin shell of individual hollow microspheres. Individual microspheres can also be opened mechanically, for instance by inserting needles into their shells.
The inventive leather is thus provided with a finish whose first layer, namely the foam-like pigmented layer, comprises hollow microspheres with a small diameter which are arranged closely adjacent one another and which brace against one another, preventing bursting of the thin shells under stress and escaping of the gas. This layer thus has a constant thickness at all points. It has been shown that even in eight hours of milling the closed cells formed by the microspheres are not destroyed, and hardly any gas escapes. Because the contact between the individual spheres is substantially only point-to-point, cavities emerge, which form open cells in this first layer which ensure the required water vapor permeability and breathability, the number of which can be further increased if necessary by opening the shells of individual microspheres as described.
The second layer, a varnish layer containing a dulling agent, advantageously comprises a thickness between 0.02 mm and 0.05 mm and is therefore thin enough that the water vapor permeability is not adversely affected by this second layer. This second layer fills the micropores at the surface of the first layer but without penetrating into this layer, so that on one hand an inseparable anchoring between the two layers is guaranteed, and on the other hand a compromising of the elastic characteristics of the first layer is avoided.
The inventive leather has a water vapor permeability of more than 0.6 mg/cm2xc3x97h, preferably of greater than 1.0 mg/cm2xc3x97h and a softness grade of at least 4.5 as a result of milling, without large areas becoming pipey. The softness grade is measured as mentioned above in the foregoing description. The thickness of the inventive napa cowhide equals between 1.0 mm and 1.4 mm.
It has proven advantageous for at least one of the layers to contain a fine wood flour in a quantity between 0.5% and 9% by volume (vol %), preferably in a quantity between 1.5% and 7%. The water vapor permeability of the finish can thus be improved, that is to say modified by varying this quantity and thereby adapted to the respective requirements.
It is particularly advantageous when the leather has the shape of a format cut prior to the applicant of the layers, i.e. when smaller portions corresponding to the utilization of the leather are cut out of a large leather skin before the finish is applied. That way, the different textures of a whole animal skin of 50 m2 which has different thicknesses in individual regions can be better accounted for. The skin is less thick in the periphery of a whole animal skin, and therefore a lower pressure can be used for the embossing, whereas the thickness is greater in the interior region of the skin.
Furthermore, smaller embossing rolls can be used for embossing format cuts, which substantially reduces the costs of acquiring these rolls, namely because the dimensioning and bearing for embossing rolls with a length of 3 meters, for example, such as those used for embossing a whole animal skin, have to be designed substantially stronger than for small embossing rolls with a length of 80 cm, for example, such as those used for format cuts.
The inventive method for producing the full grain napa cowhide is as follows: a polyurethane and/or polyacrylate based dispersion which preferably contains a pigment is applied on the grain side of the cowhide. The dispersion contains hollow microspheres with a diameter between 15 xcexcm and 45 xcexcm and/or compact particles from which such microspheres are formed in situ by the addition of heat. The microspheres have a thin shell consisting of polyvinylidene chloride and polyacrylnitrile which contains a heavy gas, preferably isobutane. The quantity of the dispersion is selected such that, after it is stabilized by dehydration, a first layer with a thickness between 0.015 mm and 0.04 mm is formed, which comprises primarily closed cells consisting of the microspheres but also at least 10% open cells by volume. The stabilized first layer is embossed by means of pressure and/or heat. A cross-linkable polyurethane dispersion, preferably containing a dulling agent, is applied thereon, preferably in several successive coatings, forming a second varnish layer The leather undergoes a milling treatment.
With the embossing of the first layer with the aid of pressure and heat, the shells of some of the microspheres are ruptured given an embossing temperature below 120xc2x0 C. as a consequence of the low distortion temperature of the microspheres, whose shell consists of at least 75% polyvinylidene chloride, and these form additional open cells, which enhance the breathability. At the same time, an inseparable anchoring to the full grain cowhide is guaranteed.
In order to destroy the shells of individual microspheres and thereby increase the number of open cells, the thin shells of individual microspheres can also be inventively dissolved by a solvent. Suitable solvents are ethylacetate or methylethylketone contained in the second layer. A mechanical destruction of the shells of individual microspheres is also possible, namely by producing holes in the first layer by inserting needles or the like, potentially during the embossing, whereby the thin shells of individual microspheres are opened. The insertion can be performed by means of a needle board or roller, and the needles can be cold or warm. After the needles are removed, the holes close owing to the elastic material of which the first layer consists, but the opened microspheres and the open cells they form remain intact.
When silicon dioxide, preferably in a particle size between 2 xcexcm and 4 xcexcm, is utilized as the dulling agent, the shells of individual microspheres can be opened by the action of this dulling agent.
The embossing expediently occurs with the aid of an embossing roll with a temperature between 80xc2x0 C. and 120xc2x0 C., whereby the thermal contact between the embossing roll and the first layer lasts less than 2 seconds. The surface of the embossing roll comprises depressions corresponding to the napa grain structure that is to be produced, into which portions of the first layer penetrate (by foaming) during embossing, so that the desired napa graining emerges.
The varnish application which is applied after the embossing can be performed in several steps, for instance by successive spraying onto the stabilized first layer and effectuates the desired color tone of the leather given corresponding dulling and pigmentation.
Besides the napa embossing, in order to achieve the typical soft fulled napa effect and to get the required softness grade above 4.5, the leather is inventively subjected to milling treatment for a period of at least two hours, preferably between 8 and 12 hours. This milling treatment can occur after the first layer is stabilized already, or not until the second layer is stabilized. It is also possible to perform part of the milling after the stabilizing of the first layer, and the rest after the stabilizing of the second varnish layer.
When the varnish layer consists of several coatings, it is also possible to perform the embossing after the first coating is applied but before the last coating is applied.
The effect of the closed cells that are formed by the hollow microspheres is that the inventive soft fulled napa leathers are only slightly pipey, if at all, and they have a high grain stability, and so the usable area of an inventive leather skin is some 50% higher than that of the known napa cowhides. The open cells guarantee the required water vapor permeability. Furthermore, open cells can be achieved by introducing air into the first layer. It has been shown that the water vapor permeability is already more than 0.6 mg/cm2xc2x7h given a 10% share of open cells by volume.
The water vapor permeability can be further improved (i.e. controlled) by admixing fine wood flour in a quantity between 0.5% and 9% by volume (preferably in a quantity between 1.5% and 7% by volume) with at least one dispersion which forms one of the two layers prior to application. The water vapor permeability can be adapted to the respective requirements by varying the amount. Thus, the disadvantageous perforating of the finished leather in the seat area of a car seat can be avoided by adding a corresponding quantity of wood flour.
Particularly when the leather will be under high stress, as with seats in public transportation, it is advantageous to subject the grain side of the cowhide to a pretreatment before the dispersion that forms the first layer is applied, by means of which the finish can be more effectively joined to the grain side of the leather. This type of pretreatment can be performed by roughening the grain side of the cowhide, preferably by abrasion. Gentle grinding with an extra-fine abrasive paper substantially improves the adhesive force, while the grain structure of the leather remains at least 90% intact.
But this can also be achieved by singeing (flame-treating) the grain side of the cowhide prior to the application of the finish. It has been discovered that this singeing produces radicals more or less in the surface, which also substantially improves the adhesion of the finish. The best flame for this is one like a soldering flame which heats the grain layer only superficially to over 160xc2x0 C., whereby the temperature does not rise above 110xc2x0 C. in the middle and on the bottom side of the leather. This treatment of the leather does not change the appearance of the grain side.
Format portions are expediently removed from a cowhide skin prior to the application of the dispersion forming the finish. That way, not only is it possible to account for the different textures of the skin in the individual regions equally and thereby guarantee the utilization of the whole skin as described above, but it is also possible to easily change the composition and texture of the finish according to requirements. For instance, the individual format portions can be designed in such a way that one portion of the finish is softer and another is harder, or such that the portions of the finish are different colors. This can be achieved easily by controlling the application of the dispersion that forms the finish by means of a storage medium such as a CD-ROM, diskette, or memory chip. Another advantage of using such format portions is that cheaper spraying systems and finer spray nozzles can be utilized for applying the dispersion forming the individual layers, and so less overspray is needed and less waste is produced.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a full grain napa cowhide and a related production method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.